The present invention relates to an image forming apparatus which performs image density control.
Generally, in an image forming apparatus utilizing an electro-photographic printing method, an image density largely varies depending on various conditions such as an environmental change of where the apparatus is placed or the number of pages to be printed. Therefore, conventionally a toner image for density detection (hereinafter referred to as a patch) with a maximum density (Dmax) is formed on a photosensitive drum or the like and the density is detected by an optical sensor. Then, the detected result is fed back to image forming conditions such as a developing bias, to perform maximum density control (Dmax control) which utilizes a Dmax value as a predetermined value. By this method, an image having a stable density can be obtained.
The Dmax control will be described below.
An image density control circuit which constitutes an image forming apparatus activates a pattern generation circuit to generate an image signal, which is indicative of a density detection patch. By following the signal, latent images of four patches P1 to P4 are formed along a driving direction of a photosensitive drum. These latent images are then developed by a developing device. Herein, a developing bias potential (V DC) is changed for every patch by a high-voltage control circuit, e.g. V1 for the first patch P1, V2 for P2, V3 for P3, and V4 for P4; accordingly, each patch is developed in different developing bias. Density values D1 to D4 for each of the formed patches P1 to P4 on the photosensitive drum are measured by a density sensor.
When a latent image of a density detection patch is developed in different developing bias V DC, the relationship (V-D characteristic) between a developing bias (V DC) and a density (O.D.) of a patch at a normal temperature and normal humidity can be illustrated as shown in FIG. 9. As apparent from FIG. 9, the V-D characteristic consists of portions A and C, where the characteristic shows a little variance, and a portion B where the characteristic shows a considerable variance. The V-D characteristic also changes, for instance as shown in FIG. 10, depending on an environment where an image forming apparatus is placed. In the chart of FIG. 10, the characteristic curve indicated as a is identical to that of FIG. 9; b is a characteristic curve under a high-temperature and high-humidity environment; and c is a characteristic curve under a low-temperature and low-humidity environment.
As shown in the V-D characteristic illustrated in FIG. 9, variance of the density in the portions A and C is unstable, but the density in the portion B shows a stable increase. Therefore, for the Dmax control, a control target DTarget is set at the portion B as shown in FIG. 10. Relationships among density values D1 to D4 for each patch is D1&lt;D2&lt;D3&lt;D4 and the developing biases V1 to V4 are set so that DTarget comes virtually to a mid-portion of D1 and D4. Values V1 to V4 are so selected that the DTarget is within a range of D1 to D4 even if the values D1 to D4 change due to a minor variance of the V-D characteristic. Also, for the purpose of simplifying a forthcoming calculation, the values V1 to V4 are set so that the difference between V4 and V3, V3 and V2, V2 and V1 (indicated as w in FIG. 10, and approximately 50 V) are all equal.
Since the V-D characteristic can vary largely depending on an environment as described above, there is a case where the DTarget exceeds the range of D1 to D4 when the values V1 to V4 are fixed (see b and c in FIG. 10). To cope with this situation, the developing biases V1 to V4 are set to vary depending upon an environment such as high temperature and high humidity or low temperature and low humidity as shown in FIGS. 11 and 12 respectively, so that the DTarget comes virtually to the mid-portion of D1 and D4.
For the above described developing biases V1 to V4, when a Dmax control is initiated, appropriate values for the Dmax control in an environment at the time the control is initiated are selected based on absolute moisture quantity of an image forming apparatus obtained from a temperature-humidity sensor embodied in the apparatus. Then, the most appropriate developing bias VTarget is computed in an image density control circuit to realize a density with respect to the control target DTarget, by utilizing data for the density values D1 to D4 of each patch measured by the density sensor and the developing biases V1 to V4 at the time when the patch is formed.
A method of computing the most appropriate developing bias begins with seeking a section, within D1 to D4, which includes DTarget, i.e. a section which satisfies Di.ltoreq.D.ltoreq.Di+1 (i is either 1, 2 or 3). When such section is found, a developing bias VTarget for obtaining a DTarget is computed by a linear interpolation. For instance, as shown in FIG. 13, when a density DTarget, which is a control target, is positioned in-between D2 and D3, the most appropriate developing bias VTarget is computed by the following equation. EQU VTarget={(V3-V2)/(D3-D2)}.times.(DTarget-D2)+V2 (1)
Then, the computed developing bias VTarget is stored in a memory and utilized for forming an image until next Dmax control is performed.
However, in the above described image forming apparatus, the density DTarget must be within the range of D1 to D4 in order to compute the developing bias VTarget. When the DTarget exceeds that range, an error processing such as selection of a predetermined developing bias as a default is performed.
Herein, the developing bias as a default is an intermediate value between V1 and V4, or V1 if DTarget&lt;D1 holds, or V4 if D4&lt;DTarget holds.
Although an image of minimum quality is ensured in this case, it is impossible to obtain an image having a stable density. In order to perform accurate image density control, it is necessary to set an appropriate developing bias by altering the developing biases V1 to V4 according to an environment where an image forming apparatus is placed, as explained above. However, even if an appropriate developing bias is set, there is a problem that a DTarget exceeds the range of D1 and D4 due to a decrease in durability of toner, an acute change of an environment which cannot be followed by the temperature-humidity sensor, or deterioration of the photosensitive drum.
Accordingly, in order to prevent such problem from occurring, a method can be thought of: such as widening a range w for each of the developing biases V1 to V4, or increasing the number of patches to more than five so that the range of a developing bias which can be controlled by the Dmax control is extended. However, the method of widening the range of the developing bias increases errors caused by a linear interpolation and the method of increasing the number of patches increases consumption of toner.